1. Field of the Invention
The present invention relates generally to a system and method for cancelling expansion waves in a wave rotor, or more specifically, to a system and method for cancelling expansion waves generated by the release of working fluid from the wave rotor using special ports.
2. Description of the Related Art
The wave rotor concept has existed since at least the 1940,s. Presently, the exploitation of the advantages provided by wave rotors has been minimal. It is believed that only one company, Brown Bovari, has developed a wave rotor successfully, such wave rotor being used as a super charger for an automobile engine.
A considerable amount of gas turbine research is focused currently on improving the efficiency of aircraft turbofan engines. One way to achieve high efficiency is by significantly raising the compressor pressure ratio (compressor exit pressure divided by inlet pressure), and/or the temperature in the burner of the aircraft turbofan engine above the values presently used. However, several constraints presently inhibit the realization of aircraft turbofan engines with significantly-raised compressor pressure ratios or temperatures. Raising the pressure ratio and, therefore, an associated gas density, requires the use of very small passages and blading in the turbofan engine. However, the increased surface area resulting from the use of very small passages and blading, causes a decrease in component efficiency through friction losses. Hence, the theoretical improvement in cycle efficiency achieved by raising the compressor pressure ratio is more than negated by the decrease in component efficiency.
On the other hand, significantly increasing the temperature of the working fluid used in a turbine is limited by the temperatures which the materials in the turbine can withstand. Presently, it is doubtful that present cooling techniques and materials would be adequate to accommodate significantly increased temperatures in an aircraft turbofan engine.
One proposed concept which may have the potential to overcome one or possibly both of these problems is the wave rotor. Contemplation of wave rotors for use as high-pressure gas turbine cores is a relatively recent development, and it is believed that no such wave rotor has ever been successfully developed. Generally, the wave rotor functions like conventional gas turbines to compress, heat and expand a working fluid to extract useful work. In a wave rotor, however, unlike a conventional gas turbine, the working fluid gains or loses energy by using unsteady, one dimensional waves instead of by using conventional rotating airfoils. This characteristic of the wave rotor reduces the problem of small blading losses present in conventional core engines. Further, the wave rotor has the distinct advantage of effectively concentrating both a compressor and a turbine in one device.
FIG. 1 is a simplified drawing of a wave rotor. The wave rotor includes a series of cells in an annular ring which rotate about an axis parallel to the cells. As the cells rotate, the ends of each of the cells are periodically exposed to various ports which create traveling compression or expansion waves due to the different states of the working fluid present in such cells relative to the state of the working fluid present outside such cells.
One requirement for successful operation of the wave rotor is to place the ports such that after opening and creating a compression or expansion wave, such ports close before any reflected waves or waves from other port openings arrive. Thus, from the perspective of the ports, working fluid flowing into or out of the wave rotor is completely steady so that the waves appear to be stationary. From the perspective of an individual cell, however, the waves are dynamic. This is illustrated by the solid lines of FIG. 1 which represent wavefronts which separate the various flow regions.
The other important aspect of successful operation of the wave rotor is that the journey of a cell through one revolution of the wave rotor must be periodic. That is, the state of the working fluid in the cell must be the same at the end of a revolution as at the beginning. Such requirements present a fundamental challenge in the design of wave rotors.
To meet these requirements, a wave rotor must account for expansion waves generated in the wave rotor by release of working fluid from the wave rotor. One known device which effects expansion wave cancellation is disclosed in U.S. Pat. No. 3,082,934 to D. B. Spalding (hereinafter "the Spalding device") issued Mar. 26, 1963, which is incorporated herein by reference. The Spalding device uses plural subsidiary ports arranged along the head and tail of an expansion wave generated by the release of working fluid through a port. While the Spalding device is believed to effect cancellation of the expansion wave, it would be desirable to provide a wave rotor which avoids the complication presented by the provision of such plural subsidiary ports and associated ducting.